1. Field of the Invention
The present invention generally relates to a control device for a hydraulic automatic transmission, and more particularly to a control device for a hydraulic automatic transmission for performing start control such that a shift position is switched through a predetermined high-speed position to a start position in performing a select operation from a nonforward range such as a neutral range (N range) and a reverse range (R range) to a forward range such as a drive range (D range). In this specification, the switching control from the N range to the forward range is referred to as in-gear control.
2. Description of the Related Art
In general, an automatic transmission mounted on a vehicle includes a torque converter, a shift mechanism, and a plurality of hydraulic engaging elements (frictional engaging elements) such as clutches and brakes for switching power transmitting paths in this shift mechanism. These hydraulic engaging elements are engaged automatically according to an operational condition of the vehicle or selectively according to the switching of ranges by a manual operation by a driver, thereby obtaining a desired shift position. In this kind of automatic transmission, the engagement or disengagement of each hydraulic engaging element is controlled by supplying or discharging a hydraulic pressure.
In this kind of automatic transmission, there is a problem that an uncomfortable shock called an ND shock may occur in performing a select operation from the neutral range to the drive range at starting the vehicle. More specifically, in such an N-D select operation, a neutral condition where all the hydraulic engaging elements are disengaged is switched to a drive condition where a predetermined hydraulic engaging element is engaged to obtain a first-speed position as a shift position. At this time, the output torque of an engine is transmitted to wheels at an increased ratio corresponding to the reduction ratio of the first-speed position, so that this large torque transmission causes the uncomfortable shock to passengers in the vehicle.
In other words, while the in-gear control is the control for switching the neutral range as an unloaded condition to the forward range (or reverse range), the input torque to the transmission at this time is small and the ratio of transmitted torque fluctuations to changes in engagement capacity of the hydraulic engaging element is large, resulting in the occurrence of the uncomfortable shock. Therefore, greatly delicate control is required for the engagement control of the hydraulic engaging element in performing the in-gear control. Various control methods on the in-gear control have been proposed in the prior art. For example, in switching the neutral range to the forward range, a high-speed shift position is temporarily set and a first-speed position (starting shift position) is thereafter set.
This control is referred to as squat control, whereby the output torque change in switching the neutral range to the forward range can be smoothened to thereby reduce a shift shock in performing the in-gear control. For example, in a shift control method disclosed in Japanese Patent Publication No. Hei 3-6390, a high-speed shift position is once set and a first-speed position is thereafter set when a manual shift lever is moved from the neutral range to the forward range in the condition where a depression force applied to an accelerator pedal has been removed and a parking brake is applied at a vehicle speed near zero.
In this kind of automatic transmission, the engagement or disengagement of each hydraulic engaging element (frictional engaging element) is controlled by a hydraulic pressure. Accordingly, at a low temperature, the viscosity of a working fluid increases to cause an increase in time required for switching the shift position to the starting position. As a result, there is a possibility that the starting characteristics of the vehicle may be degraded. A control device solving this problem has been proposed in Japanese Patent No. 3266320. In this control device for an automatic transmission, a high-speed position setting operation is inhibited at starting when the temperature of a power unit as detected by temperature detecting means is lower than a predetermined value.
The viscosity of the working fluid decreases with an increase in temperature, causing a reduction in controlled hydraulic pressure in response to a control command value. Accordingly, it is difficult to stably supply the working fluid to the hydraulic engaging elements for a plurality of shift positions. Thus, the conventional control device has such a problem that the starting characteristics at a high temperature are degraded. Further, this problem occurs also in the case that the high-speed shift position is used at a predetermined frequency or more in a given period of time to cause an increase in temperature of the hydraulic engaging element such as a clutch.